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The message does not say the alleged plotters belonged to the Islamic State group, but the advisory is described as a follow-up to a warning published in January that focused on online threats from the Sunni fundamentalist group, which has seized large swaths of Iraq and Syria and beheaded a number of Americans.

The Islamic State group is also known as ISIS or ISIL.

The Northern Command warning read, “No timetable had been established for the plot and the individuals had not yet settled on a specific target or method. However, it is evidence that (Defense Department) personnel in the homeland remain viable targets in the eyes of our adversaries.”

Also on Monday, U.S. Central Command posted an advisory to military service members warning them to be careful with information they post online.

The warning followed a threat posted last weekend by a group claiming to be the Islamic State Hacking Division that encouraged Islamic State followers to kill about 100 U.S. military service members it named.

The Pentagon said the Islamic State group apparently gleaned from social media the information about the location of the military service members it identified.

“The information that was posted by ISIL was information taken from social websites and publicly available,” Defense Secretary Ash Carter told reporters. “It wasn’t stolen from any (Defense Department) websites or any confidential databases.”

He said the Pentagon, nonetheless, takes this event seriously. Other officials have said it is being investigated by the FBI.

“At the same time, this is the kind of social media messaging of a vile sort that ISIL specializes in” and is one reason the U.S. is determined to defeat the group, he added.

Joint Base Lewis-McChord broadcasted the advisory to military families in the South Sound.

“We’re constantly reassessing our efforts to safeguard our community in response to world events,” base spokesman Joe Kubistek said. The base does not describe changes to its security posture.

Representatives from Army and Navy recruiting agencies in Seattle confirmed that they had received a warning from the Defense Department.

The Army’s Lt. Col. Vylius Leskys said his recruiters take part in anti-terrorism training.

“With the heightened concerns, our workforce maintains even greater vigilance in awareness and posture,” he said.

John Lill, a spokesman for Navy recruiting in Seattle, said, “There haven’t been any further notifications specific to our service branch, but we are focusing on being vigilant and cautious at this time.”

Members of the Islamic State group have posted threats to Westerners on online forums since last summer, when it began killing journalists and aid workers kidnapped in Syria.